66 research outputs found
The Röntgen interaction and forces on dipoles in time-modulated optical fields
The Röntgen term is an often neglected contribution to the interaction between an atom and an electromagnetic field in the electric dipole approximation. In this work we discuss how this interaction term leads to a difference between the kinetic and canonical momentum of an atom which, in turn, leads to surprising radiation forces acting on the atom. We use a number of examples to explore the main features of this interaction, namely forces acting against the expected dipole force or accelerations perpendicular to the beam propagation axis
Vacuum Friction
We know that in empty space there is no preferred state of rest. This is true
both in special relativity but also in Newtonian mechanics with its associated
Galilean relativity. It comes as something of a surprise, therefore, to
discover the existence a friction force associated with spontaneous emission.
he resolution of this paradox relies on a central idea from special relativity
even though our derivation of it is non-relativistic. We examine the
possibility that the physics underlying this effect might be explored in an ion
trap, via the observation of a superposition of different mass states.Comment: 8 pages, 2 figures. Published in Journal of Modern Optics on 14
September 2017. Version 2 with a corrected typo on page
Self-ordering and collective dynamics of transversely illuminated point-scatterers in a 1D trap
We study point-like polarizable particles confined in a 1D very elongated
trap within the evanescent field of an optical nano-fiber or nano-structure.
When illuminated transversely by coherent light, collective light scattering
into propagating fiber modes induces long range interactions and eventually
crystallisation of the particles into regular order. We develop a simple and
intuitive scattering-matrix based approach to study these long-range
interactions by collective scattering and the resulting light-induced
self-ordering. For few particles we derive explicit conditions for
self-consistent stable ordering. In the purely dispersive limit with negligible
back-scattering, we recover the prediction of an equidistant lattice as
previously found for effective dipole-dipole interaction models. We generalize
our model to experimentally more realistic configurations including
backscattering, absorption and a directional scattering asymmetry. For larger
particle ensembles the resulting self-consistent particle-field equations can
be numerically solved to study the formation of long-range order and stability
limits
Attractive Optical Forces from Blackbody Radiation
Blackbody radiation around hot objects induces ac Stark shifts of the energy
levels of nearby atoms and molecules. These shifts are roughly proportional to
the fourth power of the temperature and induce a force decaying with the third
power of the distance from the object. We explicitly calculate the resulting
attractive blackbody optical dipole force for ground state hydrogen atoms.
Surprisingly, this force can surpass the repulsive radiation pressure and
actually pull the atoms against the radiation energy flow towards the surface
with a force stronger than gravity. We exemplify the dominance of the
"blackbody force" over gravity for hydrogen in a cloud of hot dust particles.
This overlooked force appears relevant in various astrophysical scenarios, in
particular, since analogous results hold for a wide class of other broadband
radiation sources
Local Retrodiction Models for Photon-Noise-Limited Images
Imaging technologies working at very low light levels acquire data by attempting to count the number of photons impinging on each pixel. Especially in cases with, on average, less than one photocount per pixel the resulting images are heavily corrupted by Poissonian noise and a host of successful algorithms trying to reconstruct the original image from this noisy data have been developed. Here we review a recently proposed scheme that complements these algorithms by calculating the full probability distribution for the local intensity distribution behind the noisy photocount measurements. Such a probabilistic treatment opens the way to hypothesis testing and confidence levels for conclusions drawn from image analysis
Will a decaying atom feel a friction force?
We show how a simple calculation leads to the surprising result that an excited two-level atom moving through vacuum sees a tiny friction force of first order in v/c. At first sight this seems to be in obvious contradiction to other calculations showing that the interaction with the vacuum does not change the velocity of an atom. It is yet more surprising that this change in the atom's momentum turns out to be a necessary result of energy and momentum conservation in special relativity
From retrodiction to Bayesian quantum imaging
We employ quantum retrodiction to develop a robust Bayesian algorithm for reconstructing the intensity values of an image from sparse photocount data, while also accounting for detector noise in the form of dark counts. This method yields not only a reconstructed image but also provides the full probability distribution function for the intensity at each pixel. We use simulated as well as real data to illustrate both the applications of the algorithm and the analysis options that are only available when the full probability distribution functions are known. These include calculating Bayesian credible regions for each pixel intensity, allowing an objective assessment of the reliability of the reconstructed image intensity values
Mass-energy and anomalous friction in quantum optics
The usual multipolar Hamiltonian for atom-light interaction features a
non-relativistic moving atom interacting with electromagnetic fields which
inherently follow Lorentzian symmetry. This combination can lead to situations
where atoms appear to experience a friction force, when in fact they only
change their internal mass-energy due to the emission or absorption of a
photon. Unfortunately the simple Galilean description of the atom's motion is
not sufficient to distinguish between a change in momentum due to acceleration
and a change in momentum due to a change in internal mass-energy. In this work
we show how a low-order relativistic correction can be included in the
multipolar atom-light Hamiltonian. We also give examples how this affects the
most basic mechanical interactions between atoms and photons
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